This disclosure relates to a lightning arrester device for protecting an electrical circuit connected to a low-voltage network against transient overvoltages, of the type comprising at least one protective element consisting of a gas-type spark gap.
Gas-type spark gaps are elements which normally have a very high insulation resistance, which it is possible to regard as being almost infinite, and which trip abruptly and become conducting with a very low resistance, akin to a short circuit, so as to be able to divert a strong discharge current to earth when they are subjected to transient overvoltages, the value of which exceeds a certain threshold (trip voltage of the spark gap). It is thus possible to protect electrical circuits situated downstream of the spark gap against transient overvoltages which may have diverse origins, such as for example lightning, industrial disturbances, etc. For example, for low-voltage electrical circuits, that is to say electrical circuits operating under voltages of the order of 230/400 volts, one will choose gas-type spark gaps having an A.C. trip voltage of the order of 300 to 600 volts, the trip voltage chosen being, of course, slightly greater than the normal operating voltage of the electrical circuits to be protected.
Gas-type spark gaps have a surge current rating which is more or less limited depending on their construction, that is to say they are capable of allowing through, without destruction, a greater or lesser current surge wave exhibiting standardized characteristics. The surge current rating is usually defined by a maximum discharge current, most often expressed in kiloamperes (kA) and by a wave form, itself defined by two numbers which correspond respectively to the mid-amplitude rise time and fall time of the surge wave, these two times usually being expressed in microseconds (μs). By way of example, the gas-type spark gaps most commonly used to protect low-voltage electrical circuits have a surge current rating (maximum discharge current) corresponding to 20 kA, 8/20 μs, or else 5 kA, 10/350 μs.
In service, when a current surge wave appears, if this wave exhibits characteristics below the surge current rating of the spark gap activated, the electric arc struck in the spark gap will be extinguished without damaging the spark gap. The basic characteristics of the latter (static trip voltage, insulation resistance, etc.) will therefore not be modified, so that the spark gap will again be able to fulfill its role as a protective element when another transient overvoltage occurs. On the other hand, if the current surge wave exhibits characteristics above the surge current rating of the spark gap activated, there is a high risk that the electric arc struck in the spark gap will damage the latter. If such is the case, the trip voltage of the spark gap will rise steeply. For example, it will go from 300 volts to 700 volts or more, so that, if a transient overvoltage whose amplitude is below the new trip voltage of the spark gap then occurs, the latter will no longer be able to fulfill its role as a protective element. The electric arc may even damage the spark gap to such a point that it is subsequently totally incapable of tripping in the event of an overvoltage (non-tripping state).
Transient overvoltages are a phenomena which most often occur in a manner which is completely unpredictable in time, and whose intensity cannot be predicted either. This is why, in order to increase the chances of a lightning arrester device being capable of withstanding, without destruction, a high transient overvoltage and/or several transient overvoltages occurring in succession, it is sometimes desirable to be able to use a lightning arrester device having an enhanced surge rating.
An obvious solution to this problem consists in using a gas-type spark gap intrinsically having a greater surge current rating. Such gas-type spark gaps exist on the market. They nevertheless have the drawback of being much bulkier and much more expensive than gas-type spark gaps having a smaller surge current rating, such as those cited earlier by way of example. By way of comparison, gas-type spark gaps having a surge current rating of 20 kA in 8/20 μs or 5 kA in 10/350 μs consist of cylindrical elements having a diameter of 8 mm and a length of from 6 to 8 mm, whilst gas-type spark gaps which have a surge current rating of 50 kA in 8/20 μs or 15 kA in 10/350 μs consist of likewise cylindrical elements having a diameter of around 16 mm, and a length of around 30 mm. Furthermore, the first spark gaps indicated hereinabove are standard elements, which are manufactured in large numbers at a cost price of 1.5 francs each, whilst the second spark gaps, of higher surge current rating, are special elements which are manufactured in small numbers at a cost price of around 100 francs each.